Manca tutta la parte relativa al timer ... non l'hai proprio presa in considerazione ...

Guarda questo

codice:
Option Explicit

Dim mX As Single
Dim mY As Single

Private Sub Form_Load()

    With Timer1
        .Enabled = False
        .Interval = 300
    End With
    
    With Image1
        .Left = 0
        .Top = 0
        .Width = Picture1.Width
        .Height = Picture1.Height
    End With
 
    With Picture1
        .FillStyle = 0
        .FillColor = vbRed
    End With
End Sub

Private Sub image1_MouseDown(Button As Integer, Shift As Integer, X As Single, Y As Single)
    Static flag As Boolean
    
    If Button = 1 And flag = False Then     'tasto sinistro disegna un punto sul disegno
        mX = X
        mY = Y
        Picture1.Circle (mX, mY), 75, vbRed
        flag = True
        Timer1.Enabled = True
    ElseIf Button = 2 And flag = True Then  'tasto destro cancella il punto sul disegno'
        Timer1.Enabled = False
        Picture1.Cls
        flag = False
    End If
End Sub

Private Sub Timer1_Timer()
    Static st As Boolean
    
    If st Then
        st = False
        Picture1.Cls
    Else
        Picture1.Circle (mX, mY), 75, vbRed
        st = True
    End If
End Sub